Guinea pig vas deferans smooth muscle myosin can be phosphorylated by an endogenous kinase. The phosphorylated site is present on the 20,000 dalton light chain. Preliminary experments inducate that phosphorylation of this smooth muscle myosin either by its own kinase or by platelet myosin light chain kinase, lads to an increase in the actin-activated myosin ATPase activity.